Musnad Ahmad Hadith 20998

It is narrated from Jabir (may Allah be pleased with him) that there were a few white hairs on the front part of the head of the Prophet (ﷺ). When he applied oil to his head, the whiteness of the hair was not apparent, but when he did not apply oil, the whiteness became visible. The hair on the head and beard of the Prophet (ﷺ) was thick. Someone asked whether his face was shining like a sword? He replied, "No, rather it was shining like the moon and the sun, and it was round in shape." And I saw the Seal of Prophethood on the blessed back of the Prophet (ﷺ); it was the size of a pigeon’s egg and resembled the rest of his body.